The invention relates to a transfer switch which transfers electrical loads from one power source to another power source. This is usually performed automatically by transferring power from a normal electrical power source to an emergency electrical power source upon reduction or loss of voltage. The invention also re-transfers the load to the normal power source when the normal voltage has been restored within acceptable limits.
Automatic transfer switches have been used for many years in applications where it is required to have an emergency power source that can be automatically and quickly connected to a load should the normal power supply to the load fail. Automatic transfer switches of this type are generally characterized by complexity in view of the need to ensure that the load is momentarily disconnected from both power sources. This momentary interruption of power usually causes nothing more serious than a flickering of lights, and is usually of no great consequence. The automatic transfer switches of the type according to the invention necessarily provide a clear "break-before-make" sequencing of switch contacts, so that the load is momentarily isolated and the two power sources are never connected together, which is an undesirable condition.
Several types of automatic transfer switches and/or associated circuitry are disclosed in U.S. Pat. Nos. 4,157,461 to Wiktor; 4,189,649 issued to Przywozny et al.; 4,398,097 issued to Schell et al. and 4,423,336 issued to Iverson et al. Many types of automatic transfer switches are available to actuate switch toggles of conventional molded-case circuit breakers, but sometimes the circuit breakers are not easily adaptable to be actuated by particular automatic transfer switches. Some of the automatic transfer switches have complex cam mechanisms to provide fine adjustment for opening the circuit breaker, and subsequent closing of the remaining circuit breaker, so as to ensure the clear "break-before-make" sequence of operation. The means to provide this adjustment results in complexity, and requires some skill in setting up the transfer switch to ensure reliable operation. The necessary adjustment to provide the correct sequence is time consuming and is subject to human error.
In some automatic transfer switches, a motor is required to rotate the cam mechanism which actuates the switch toggles. The motor rotates the cam through a precise angle during the operation of the transfer switch, and with some designs the cam is required to stop in a critical position after complete actuation of the transfer switch. This often requires a brake on the motor or an escapement means which allows for disconnection of the motor from the cam mechanism so that "over-travel" of the motor is isolated from the cam rotation. The brakes and/or escapement means of the prior art transfer switches increase complexity and require additional time for maintenance and checking, which must be performed periodically.
Furthermore, when servicing such transfer switches provided with motor-driven cams, it is convenient to provide a manual operation mode wherein the motor drive and the cam means can be disconnected to permit manual rotation of the cam. Also, for servicing, it is necessary to sometimes isolate the load from both power sources, and both of the requirements above tend to increase complexity of prior art automatic transfer switches.
Also, some prior art transfer switches have a relatively short period or "operating differential" betwee breaking contact with one power source, and making contact with the remaining power source. Some prior art transfer switches are not easily adjustable to increase the period during which the load is isolated and this can present difficulties with the type of electrical load which re-generates electricity immediately subsequent to disconnection from the source. Electrical motors, when disconnected from a first power source, immediately re-generate electricity, and when the new or second power source is to be connected, an out-of-phase connection to the second power source may cause damage to equipment. Usually, the said re-generation is of a very short duration, and problems associated with out-of-phase re-closing can be reduced if the load can be de-energized for a substantial period of time, for example greater than 0.5 seconds. Transfer switches which would otherwise operate with relatively short periods where the load is isolated or de-energized consequently require either a pause in midtravel, or means to detect phase of the two sources prior to connection, so that the load is transferred only while the two sources are in phase. Both of these solutions to out-of-phase reclosing problems increase complexity and reduce reliability of the transfer switch due to introduction of additional control devices.